1. Field of the Invention
This invention relates to improved hybrid optical recording discs, together with apparatus and methods for manufacturing them.
2. Description of Prior Art
CD-Rs (recordable compact discs) and DVD-Rs (recordable digital versatile discs) are well known in the art. A less familiar new format, DVD+R has recently been introduced, but practitioners of ordinary skill in the art are, or will soon be, familiar with this format as well. In any of these formats, data, represented in an extensive sequence of elongated, three-dimensional marks in a spiral track winding circularly around the disc, may be optically recorded by rotating the disc at constant linear velocity (“CLV”) and directing a selectively-controlled laser beam at a pre-grooved track in a recording layer provided near one of its essentially planar surfaces. For simplicity, the ensuing discussion will concentrate on the CD-R format, although occasional reference will be made to the DVD-R format. The differences are well understood by those of ordinary skill in the art, as well as the manner in which this discussion should properly be understood for application to any hybrid disc format based on representation of data by microscopic three-dimensional marks.
Also, for the sake of clarity, certain dimensional conventions will be employed: “Radial” or “transverse” will mean “radially, from or toward the center of the disc.” “Longitudinal” will mean “along the track,” e.g., from the leading edge to the trailing edge of a data mark. The longitudinal direction, at the microscopic level, will thus be normal to the radial direction, both axes being parallel to the disc surface. Accordingly, a measurement transverse to the longitudinal direction, in the plane of the disc, will be in the radial direction. “Vertical” will mean “normal to the disc surface, as well as normal to the radial and longitudinal directions.”
The recording layer comprises a dye whose color is complementary to that of the write laser, covered by a thin, reflective metallic layer (in turn, typically covered by a final, protective layer) to reflect the laser beam energy back into the recording layer. The beam is normally directed through the disc substrate (“second surface recording”), which is a suitable transparent material, typically polycarbonate, to create the data marks in the recording layer near the opposite surface.
In accordance with convention, the input data are subjected to EFM (eight-bit-to-fourteen-bit) modulation, in the case of CD-Rs. Here, sequential binary input data (to which the information to be recorded and later retrieved has been converted) are converted into a sequence of spaced rectangular pulses, each of whose durations is nT, where T is the nominal EFM clock period, approximately 231 nanoseconds (billionths of a second), and n is an integer from 3 to 11. In the case of DVD-Rs, “EFM Plus” modulation is employed. This differs from EFM modulation principally in that: (1) eight-to-sixteen bit modulation is employed; (2) the integer n may be 3 to 11, or 14; and (3) T≈38 nanoseconds. Every EFM or EFM Plus encoded data stream always contains pulses and intervening temporal spacing comprising all of the possible nT durations. As is well known in the art, the interval between each transition (pit-to-land or land-to-pit) and the next successive transition separately represents a quantum of data. Thus each data pulse and each intervening land is of nT duration, where, in each data stream, all permitted values of n must be represented in both the pulses and the intervening lands. Other modulation schemes have been used or proposed, and further modulation methods will doubtless be employed in the future, as the data density on optical recording discs inevitably increases. However, it should not be difficult to generalize from this discussion to encompass any such ordinary engineering modifications.
In the case of CD-Rs or DVD-Rs, which are the principal subject of this discussion, each data track recorded onto a spiral pre-groove in the recording layer comprises a succession of a great number of microscopic, three-dimensional marks interspersed by unmarked, or differently marked, lands. However, the present invention applies also to mastering hybrid re-writable media, such as CD-RW hybrid discs and magneto-optic (MO) discs, where the data marks (with the exception of ROM marks in CD-RW masters and replicatable marks in MO discs) would be essentially two-dimensional. CD-RW hybrid discs and MO discs will be further mentioned briefly below.
The track pitch (i.e., the radial distance between the longitudinal axes of adjacent, essentially circular track portions) is microscopic, as is the length and width of each of the marks. Superimposed on the otherwise smoothly spiral tracks is “ATIP” (absolute time in pre-groove) timing data, which, in the CD case, is contained in a radially sinusoidal carrier modulation at 22.05 kHz (nominal), having an amplitude, in respect to the longitudinal axis of the “un-wobbled” spiral pre-groove, of ±30 nanometers (nominal).
Despite its ATIP wobble and its necessarily spiral configuration, a data track in the recording layer may be thought of as a large number of closely spaced, essentially circular pathways, each containing a great many three-dimensional data marks and intervening lands in succession. In some applications, a particular data track or track portion might not be completely circular, in the sense that it might occupy only an arc of a circle on the disc. However, in this discussion arcuate and circular data tracks or portions of tracks will be referred to interchangeably as being circular data tracks. Since the circumference of each of these essentially circular pathways is very great, in comparison to the dimensions of the marks and lands, a small succession of marks and intervening lands will appear to be a linear (i.e., straight line) sequence at the microscopic level. Accordingly, at the microscopic level, radially adjacent data tracks on the disc may be viewed as essentially parallel lines of data, each containing a longitudinal succession of linear marks and lands, although at the macroscopic level they are essentially concentric circular paths.
The data marks appear as elongated, slightly bulging three-dimensional marks within the pre-groove, confined between the reflective layer and the substrate. To some degree, at least, each mark includes a distortion of both the substrate and the reflective layer. The character of the material in the recorded mark differs from the unrecorded areas of the recording layer, in that the index of refraction of the material in the mark is changed by imposition of the modulated laser beam, and additional physical and chemical changes occur as well. The intensity of the laser beam is modulated in accordance with the encoded data to be recorded, and each resulting mark and each intervening land represent a portion of the data. In CD-R recording, the run length of each data mark and land corresponds to a pulse of 3T to 11T duration. Since CLV is employed, all marks and lands corresponding to the same nT value are ideally of the same length. Once recorded, the data may later be selectively retrieved (i.e., decoded and processed) by means of a CD player. Ideally, a CD player will not be able to distinguish between data marks and lands read from a CD-R or from an ordinary CD-ROM (compact disc read only memory, e.g., software CDs), and data will thus be retrievable from each format in the same manner.
Various “write strategies” i.e., data signal modulating schemes (e.g., a leading end intensity boost of prescribed amplitude), may modify the encoded data signal in connection with creating the final laser beam intensity controlling (i.e., modulation) signal. These are intended ultimately to create data marks and lands whose leading and trailing ends are three-dimensionally symmetric, and of the proper lengths, to ensure accurate “HF” (high frequency, i.e., data) retrieval by minimizing systematic mark length errors. The latter depends on accurately measuring the length of each mark, from its leading edge land-mark transition to its trailing edge mark-land transition, and measuring the length of each land from its leading edge mark-land transition to its trailing edge land-mark transition, and reconverting these lengths to their corresponding nT values. If the marks and lands are three-dimensionally symmetric, and with proper lengths, HF retrieval is facilitated by enabling selection of a particular reflectivity, which will then correspond to either transition point.
Since CD-Rs and CD-ROMs are intended to be read interchangeably in a CD player, this write-strategy adjustment of the encoded data signal is important to ensure that the player is unable to detect any difference between a CD-R or CD-ROM, although CD-Rs must conform to the Philips-Sony “Orange Book” specification, while a CD-ROM must conform to the “Red Book” specification. These Philips-Sony specifications are well known to those of ordinary skill.
As stated above, the optical disc is normally recorded and later read by rotating it rapidly on a motor-driven spindle, at CLV. (It might be noted that some players utilize constant angular velocity playback, although this discussion will assume CLV recording and playback.) Tracking—maintenance of the radial position of the write beam and/or the read beam precisely in the center of the data track—is accomplished through a servo apparatus that compares at least a single pair of continuous readings. In CD-R players, single beam (“push-pull”: “PP”) tracking is normally employed. Here, the reflected beam is optically split into a data retrieval beam and a tracking beam. In CD and DVD-R applications, the reflected tracking beam component of essentially circular cross-section is divided into two equal semi-circles, the dividing line between them being parallel to the longitudinal track axis, i.e., each reading in the pair is taken on opposite sides of the longitudinal axis of the track. The tracking sensor continuously compares the intensity of the two halves of that image, and a servomechanism adjusts the radial position of the beam to cause the sensed light in both halves to be equal. The latter condition indicates that the readings are being taken from the center of the track axis, i.e., that proper tracking is occurring.
Generally, the same sensor is employed for tracking and for data retrieval. In single-beam CD tracking, one of the two sensed components is subtracted from the other, and a zero difference (i.e., equal input from both sides) indicates proper tracking. Data retrieval is accomplished by adding the two halves. In pre-recorded DVD applications, differential phase tracking is employed, in which the reflected light is divided into four quadrants and the phasing of each is compared to determine tracking condition.
Most CD-ROM players employ triple-beam HF retrieval and tracking, as shown in FIG. 4, discussed below (although the Red Book only prescribes standards for single-beam, PP tracking). In triple-beam tracking, the read beam is split into three beams, the read beam itself, a first tracking beam directed one or more mark lengths ahead of it and offset ¼ of the track pitch (approximately ¾ mark width) to one side, and the second tracking beam directed one or more mark lengths behind the read beam and offset ¼ of the track pitch to the other side. Each of the two tracking beam reflections is individually sensed continuously for tracking, in the manner described above in respect to PP tracking.
Beam focusing is likewise accomplished through a suitable feedback mechanism. Beam focusing is commonly employed and well known in the art, and therefore need not be further described except as may be necessary to describe particular applications.
Part II of the Orange Book contains the specifications for hybrid discs, such as described in some detail in U.S. Pat. No. 5,204,852, Nakagawa et al. In simplest terms, a hybrid disc is an optical recording disc that, in alternating annular track bands, contains pre-recorded data (the “ROM” bands), and bands of pre-grooves covered with an optical recording layer (the “R bands”) on which data may be selectively recorded in the manner described above in respect to CD-Rs. The ROM data may, for example, comprise encryption information to prevent copying of the selectively recorded data, or it might comprise instructions to the CD-ROM player as to how the recorded data should be decoded and/or processed. Of course, there are many other types of data that could be pre-recorded into the ROM region. According to the Orange Book, the hybrid disc must have five annular bands. Radially from the disc center—as is well known, optical discs are normally recorded and read from the center toward the outer circumference—and with reference to the hybrid disc 300 shown schematically in FIG. 19 (with bands of arbitrary radial width) the first is the R1 band 301, which allows the CD-R recorder to optimize its nominal write laser power. Next is the ROM1 band 302, the “PMA” (program management area), which typically comprises only up to a few disc turns. This band contains information on the number of tracks in the disc, and its purpose is to close the disc after recording. Next, in succession, are selectively wide R2, ROM2 and R3 regions, 303, 304, 305, respectively, the last of which may extend nearly to the outer circumference of the disc.
Manufacture of a hybrid disc is a multi-step process. First, a hybrid disc master must be created with the requisite sequential ROM and R bands. Here, the ROM data are recorded in an optical recording layer on a surface of a blank master, whose substrate can be any convenient material, such as polycarbonate or glass, for typical first surface (“from the top”) recording, as described below. The substrate must, of course, be transparent for disc masters created by second surface recording. The ROM data, as mentioned above, can be encryption data or anything else the manufacturer wishes to record in the ROM region(s). The R bands of the disc master will display the spiral tracking pre-groove. The entire disc master will be provided with ATIP timing information in a conventional manner.
The hybrid disc master is typically created by one of two methods, respectively the photoresist (“PR”) process or the dye-polymer process. Other disc mastering processes may exist and doubtless others will arise in the future, but this discussion will concentrate on these two methods, of which the PR process is presently the more commonly employed.
The Photoresist (“PR”) recording method is essentially a photographic engraving process. The recording surface of the hybrid disc master comprises a thin, photosensitive polymer resin layer of substantially uniform composition, which has exposure characteristics virtually identical to those of photographic film emulsions. Accordingly, PR is a purely photochemical (i.e., optical) method of optical data recording. In other words, in PR, it is not the quantity of heat instilled in a selected small portion of the disc surface that exposes the photoresist; it is merely the quantity of incident light that determines if sufficient exposure occurs to enable formation of the desired three-dimensional features.
A threshold quantity of light is required to effect initiation of exposure at the photoresist surface. The extent and depth to which the photoresist below the immediate surface is exposed depends on the intensity and duration of the impinging write laser light and the optical characteristics of the photoresist material, itself. Because of light absorption and scattering within the photoresist medium, and the development process, the width of the exposure within the medium typically decreases as its depth increases. However, as a general principle it is accurate to say that increasing the incident intensity will tend to increase the depth of exposure within the photoresist medium, assuming the same duration (rotation velocity). The radial width of the exposure (particularly at the surface) is determined by the write beam width, it being understood that the beam cross-section may be thought of as an Airy disc, with a Gaussian distribution of intensity, radially. Since PR is a purely optical process, exposure begins and ends instantly, as the write beam is, respectively, activated and deactivated for each write pulse, as the disc rotates beneath the beam.
The data pulses (“on” times) ultimately generate pits in the hybrid disc master surface, while the “off” times result in the intervening lands. Because the entire data stream is encoded, both the marks and lands contain independent data. In PR recording of a ROM area, an EFM (or EFM Plus) coded waveform results in modulation of the intensity of a focused spot of light (normally from a diode or gas laser) impinging on the recording surface of the revolving disc master. By properly synchronizing the CLV rotational speed of the disc with the radial position of the beam relative to the disc center, this produces a track of narrow, elongated latent images produced when the light spot is “on,” interspersed with unexposed lands, in the ROM bands. A continuous latent image, spiraling around the disc, is created in the R bands. The width of these images will be somewhat less than ˜1 micron, in the case of CD mastering, and approximately half that with DVD mastering, since DVD dimensions are approximately half those of CD.
When the entire ROM area and the R band spiral track (or collection of concentric tracks) are “exposed” onto its surface, the hybrid disc master is “developed,” as in the case of ordinary photographic film. In this step, an etching solution is introduced to dissolve and remove the exposed regions of resist (or the unexposed regions, depending upon whether a positive or negative resist is used), which creates a succession of a very large number of narrow, three-dimensional, elongated microscopic pits and intervening lands in the ROM bands and spiral pre-grooves in the R bands of the hybrid disc master. In the ROM bands, the width (i.e., transverse extent) of these ROM features, at the disc surface, will be essentially identical to the effective width of the write beam, as will the disc surface width of the pre-grooves in the R bands. Below the surface, the width of each pit and pre-groove will decrease. Once again, the interval between each successive pair of transitions (from a pit-land transition to the next land-pit transition, or vice versa) individually represents a quantum of data corresponding to a particular data packet (i.e., pulse) in the original EFM (or EFM Plus) signal.
The thickness of the photosensitive data layer (deposited on the much thicker glass or polycarbonate substrate) is usually selected to be identical to the desired ROM pit depth. Thus, when the photoresist is fully exposed (through its entire thickness) by a write beam of sufficient intensity, flat-bottomed pits will be produced. Their depth will be the same as the thickness of the photosensitive layer. They will display, in transverse section, an essentially trapezoidal shape, whose sidewall-to-base (and, usually, surface-to sidewall) junctions will be rather sharply angular. Because the PR method is a photoengraving process, and the photoresist will not be of absolutely uniform consistency, the sidewalls will be somewhat rough.
Failure to fully expose the photoresist (resulting in residual photoresist at the bottom of the pit) by utilizing reduced write beam intensity will create pits or grooves having an essentially triangular section. As discussed below, earlier practitioners employing the PR method to create hybrid disc masters have often generated R band pre-grooves of that transverse configuration. Such shallow features, created by the PR method, have been found generally to produce “noisy” data output readings from replicated discs, because of inherent roughness in the etched photoresist layer and greater susceptibility to write laser noise. Indeed, the PR method of disc mastering, because it is an etching process, generally produces three-dimensional features with rough surfaces. These compromise detection accuracy, because the playback signal amplitude is affected by surface characteristics. This problem will be further discussed in respect to U.S. Pat. No. 5,696,758, Yanagimachi et al., and U.S. Pat. No. 6,212,158, Ha et al.
In any event, the thickness of the photoresist layer and the exposure level—and thus the resulting pit (or pre-groove) depth—are normally selected for optimal detection from replicas in a manner well known by those skilled in the art. As stated above, pit and pre-groove width are determined by the power and effective width of the recording beam. The latter is determined by the wavelength of the write laser utilized and the numerical aperture (“NA”) of the focusing means, in a conventional manner. The transverse sectional shape of the pit, whether trapezoidal or triangular, may be controllable, to at least some degree, by the optical characteristics of the photoresist material, the etching process, the power of the write beam and the particular focusing configuration selected, as described in the patent literature. Finally, the length of each resulting ROM pit will be primarily determined by the duration of the corresponding EFM (or EFM Plus) data pulse, as will be the length of each intervening land.
Once again, regardless of the particular application, the PR method is essentially an etching process, and even if the photoresist is fully exposed, a certain amount of roughness inevitably occurs on the pit sidewall surfaces. While this has not proved to be a particularly significant problem in ordinary CD-ROM mastering applications, the PR method of disc mastering is unconducive to production of hybrid CD masters from which hybrid CDs can be rapidly manufactured with a low rejection rate. This is due to at least four inherent characteristics of the PR method: (1) it tends to generate features with sharp corners, in transverse section, causing molding problems that increase cycle time; (2) shallow features, created by partial exposure of the photoresist, are noisy when read; (3) it is a generally difficult process to control; (4) the inherent roughness of PR-generated data pits and grooves impedes accurate hybrid CD data retrieval. Furthermore, this problem of roughness in PR-generated disc masters can only become more troublesome as data densities increase beyond the present CD and DVD level of approximately 4.2 gigabytes per data layer, and data retrieval strategies necessarily become more sophisticated.
Dye-Polymer optical data recording is addressed in U.S. Pat. No. 5,297,129 (hereinafter, “the '129 Patent”) and U.S. patent application Ser. No. 09/558,071, the parent of the present application (hereinafter, the “Parent Application”), each assigned to the assignee of the present application, and both fully incorporated herein by reference.
Unlike the PR method, dye-polymer recording is a thermal process, proceeding on the basis of physical principles quite different than those underlying PR. As discussed in the '129 Patent and the Parent Application, this thermal process requires a fairly sophisticated write strategy. This is because in the dye-polymer case, a small amount of time is required, after the beam is activated at the beginning of each pit formation, as the disc master rotates, to heat the dye-polymer to its thermal threshold. This causes a tapered leading edge of the resulting pit. On the other hand, cooling occurs almost instantly when the beam is shut off, resulting in relatively blunter trailing edges. Thus, unlike the purely optical PR case, where virtually unmodified EFM pulses may be utilized for laser beam intensity modulation, dye-polymer disc mastering requires careful modification of the EFM pulses to counteract these thermal effects. The '129 Patent addresses this problem and teaches an effective dye-polymer mastering write strategy, while the Parent Application non-exclusively identifies various equivalents.
Hybrid CD or DVD mastering by the dye-polymer process comprises selective expulsion of the photo-thermally active recording layer of the hybrid disc master, to yield a succession of pits and intervening lands, each representing data, in the ROM bands, and yielding pre-grooves in the R bands. This recording layer comprises a mixture of a polymer (e.g., nitrocellulose) and a dye whose color is complementary to that of the (typically, laser) write beam to promote maximum heat absorption. The proportion of dye in the dye-binder mixture should be sufficient to obviate the need for excessive write laser power, while low enough to minimize the effects (e.g., noisy readings from the final hybrid disc) that might result from dye residue on the surface of the hybrid disc master. The proportion of dye in the dye-polymer mixture is generally quite low, with a preferred range of approximately 3–5%. As more powerful, narrower write beams, e.g., ion or electron beam sources, are utilized, the selection of the dye color (if, indeed, any dye is even required in such applications) would proceed according to generally understood principles, based on the particular type of write beam selected.
Accurate data retrieval requires accurate tracking. So the recording parameters employed in dye-polymer hybrid disc mastering must provide sufficiently precise three-dimensional pit shapes, land configurations and pre-grove profile, to ensure that commercial CD and DVD players can accurately follow the data track(s) in the replicated and subsequently recorded hybrid discs, while they perform accurate HF data retrieval. Unfortunately, this is complicated by the fact that the criteria inherent in accurate HF detection and in accurate “PP” (push-pull) tracking, required by the Red Book specifications in all pre-recorded CD applications, are mutually exclusive. There exists a similarly fundamental tradeoff in PP and groove reflectivity, in the Orange Book CD-R, DVD-R and hybrid disc specifications.
The desired HF optimization is achieved with an effective pit depth (noting that each pit will normally posses a curved base, caused by the dye-polymer expulsion processes) equal to λ/4, where λ here is the wavelength of the (typically, laser) read beam within the substrate material (since discs are normally read from the second surface). This will create a π(180°) phase shift in the reflected light, effectively canceling out, by interference, the small proportion of incident light not already scattered away by diffraction. By contrast nearly 100% of incident light is reflected from the essentially flat land areas. It can easily be seen, then, that with λ/4 effective pit depth the change in reflected light detected at each pit/land transition will be very abrupt, thus facilitating accurate detection of pit and land length—i.e., accurate HF detection.
By contrast, PP detection generally measures the quantity of light diffracted from the pit at an angle in respect to the vertical direction. This is normalized with the known or observed reflectivity of the disc surface, to provide comparative values in the particular context. Thus, in CD applications, radial PP detection is merely an amplitude comparison of detected light on either side of the longitudinal track axis (whether within a pit, land area). When more reflected light is received on one side of the PP detector than the other, the PP servo moves the read beam in the opposite direction, radially, until detection in the two halves is equalized, indicating proper tracking. A λ/8 effective groove depth, producing a π/2 phase shift, optimizes PP detection, rather than the λ/4 effective groove depth and corresponding π phase shift that optimize HF detection. As mentioned above, a similar λ/8−λ/4 dichotomy exists between PP and unrecorded groove reflectivity in CD-R and DVD-R applications, and other optical recording applications, such as hybrid CDs, present closely analogous dichotomies.
A recently granted European patent—EP 96908632.1, (hereinafter, “Schoofs”)—deals, to some extent, with the problem of improving PP detection in an optical data disc (apparently, a dye-polymer recording), hopefully without unduly compromising HF detection. The proposed solution is to maintain the intensity of the write beam between write pulses at a level just above the thermal threshold of the moving medium. This creates a narrow, shallow groove in the land area connecting successive pits, which essentially increases PP tracking signal strength between pits with hopefully little negative effect on HF (i.e., pit/land transition) detection accuracy.
To a certain degree, the method taught there would superficially seem to satisfy the PP optimization criteria, in that the land groove can be made to be approximately λ/8 in effective depth (in the hybrid disc case, its effective phase depth would be approximately λ/8). However, because this is accomplished by reducing the write beam intensity to near the thermal threshold, the resulting land groove must necessarily be quite narrow. But this actually compromises PP detection, because another feature of the PP/HF dichotomy is that optimal PP detection is realized with a groove that is wider than one that would optimize HF detection. Furthermore, HF detection is not significantly addressed by Schoofs. Indeed, the logical extension of the Schoofs teachings would be to further increase beam intensity between pits to widen the groove for hopefully better tracking. But that would actually compromise PP detection by deepening the groove and would also compromise HF detection by causing pit/land transitions to be more difficult to detect, thus negatively counterbalancing any proposed PP improvements.
In the context of hybrid disc manufacture, very similar problems will also occur in following the Nakagawa, Yanagimachi and Ha teachings, to be discussed shortly.
Creating a disc master by whatever method (e.g., the PR method or a thermal method, such as the dye-polymer method), and in whichever format (CD-ROM, hybrid CD, etc.) is only the first step in the disc production process. It is the final disc that is of principal interest, not the disc master. The final disc, to which the manufacturing specifications are addressed, is not obtained until a number of intermediate steps have been taken in the manufacturing process.
Once recorded, the hybrid disc master is converted to a metal stamper by conventional galvanic processes, and polycarbonate daughter discs are molded from it. If sufficient skill and care are exercised, the stamper will be a virtually exact mirror image of the master, and the resulting “clear replica” hybrid discs will likewise be virtually exact copies of the master. They will display the recorded ROM data in the ROM bands, and will display the necessary CD-R pre-groove in the R bands. Failure to properly optimize the overall disc production line, by ongoing testing of final (processed) hybrid discs and corresponding adjustment of the mastering parameters, in a feedback loop process (see, below), may yield stampers that are reasonably good mirror images of the masters and clear replica hybrid discs whose features are closely similar to those of the master, and yet result in final hybrid discs whose ROM data marks and pre-grooves do not exactly display the cross-sectional shape of those in the master. The latter, as well as later-recorded hybrid discs, must conform to Orange Book specifications (and also to Red Book specifications, incorporated into the Orange Book, in respect to the ROM areas). There are no specifications for the hybrid disc masters or for the clear replica hybrid discs, themselves, since it is only the ultimate replicas that are of commercial interest.
Not only is it necessary to optimize the mastering process to enable production of final hybrid discs that meet manufacturing specifications, it is also necessary to maximize their Figure of Merit. As is well known in the art, the Figure of Merit is a weighted function that measures overall conformity to the applicable specifications, such as the amplitude of HF detection, the amplitude of PP detection, minimization of cross talk between radially adjacent portions of the data track, etc. The Figure of Merit is increased as the observed parametric values of the final hybrid discs within the relevant specification categories are brought closer to the center of the acceptable ranges of each of those categories, and maximized where only a lower limit applies. Thus, maximizing the Figure of Merit means that the system has been optimized to such an extent that the often-unpredictable variations normally encountered in the various manufacturing steps will probably not cause the final products to be out of spec. Maximizing the Figure of Merit, therefore, ensures a “forgiving” system and good product yields.
According to Red Book specifications, the CD track pitch (“TP”) is between 1.5 and 1.7 microns, the nominal value being 1.6 microns. The length of an EFM-coded CD pit, measured at half-depth (where all widths and lengths are conventionally measured), is nominally 0.3 micron per T, where the pit length spatially represents an input data pulse run length of nT temporal duration. The width (“PW”) of a CD pit (again, measured at half depth, as shown in FIG. 18) and the spot diameter of the write beam creating it are each approximately 0.5 micron, i.e., approximately TP/3. On the other hand, the read beam is approximately double that width, or about 1 micron wide. Since various laser wavelengths are utilized in CD recording, the numerical aperture of the objective lens focusing the beam must be selected to yield a beam spot of the same diameter regardless of the beam source, so that the pits will be the same width regardless of the apparatus used, to ensure that the resulting pits may be uniformly read. The spot diameter, d, is determined by the formula d≈0.5λ/NA, where λ is the beam wavelength in vacuo, NA is the numerical aperture and d is the diameter of the resulting spot. In the case of CD playback, for example, λ=0.780 micron, and NA=0.45, so d≈0.9 micron.
Similar proportions apply in DVD applications, although the transverse dimensions pertaining to DVD recording and reading are approximately 50% of those of CD applications, reflecting the correspondingly shorter channel bit lengths of DVD marks. Presumably, future higher-density applications—utilizing higher frequency (i.e., shorter effective wavelength) write and read beams, smaller pits and narrower track pitches—will employ similar relative proportions.
Double-sided (or layered) discs can be generated by utilizing two molds—one for each side (or layer)—each made from a separate disc master, recorded in the manner discussed above.
By whatever method the master is recorded, the final steps in hybrid disc manufacture are spin coating, onto each clear replica hybrid disc, a thermally-active recording layer; over-coating that layer with a thin, metallic reflective layer; and, normally, applying a protective layer above the latter. The recording layer will, of course, tend to fill in portions of the ROM data pits (particularly, those of shorter run lengths, e.g., 3T to 5T, as discussed below) and intervening lands, as well as the R band pre-grooves. The depth of the resulting optical recording layer above the ROM pits and the R band pre-grooves (into which the CD-R data pits will later be recorded) depends on a host of factors, e.g., the viscosity of the layer before drying, the drying conditions, the spin velocity and the transverse shapes of the pits and pre-grooves. The final hybrid discs, as well as later-recorded hybrid discs, must, in their entireties conform to the Orange Book specifications, and their ROM areas must also conform to the Red Book specifications, insofar as they are incorporated into the Orange Book specifications.
Despite confident assertions made in the prior art, their teachings do not, individually or collectively, appear capable of providing a method, apparatus or structure that offers the range of selectively adjustable parameters, nor the general flexibility, necessary to maximize Figure of Merit and thus promote reliable production of hybrid discs that satisfy Red Book and Orange Book specifications in a high-speed manufacturing environment. These prior art deficiencies will now to be discussed.
The Nakagawa hybrid disc patent (U.S. Pat. No. 5,204,852), mentioned above, is based on photoresist recording of ROM data pits and R band pre-grooves (Column 5: lines 3–20), and teaches exposure of the photoresist in the ROM area at a different level than in the R band area. Two basic embodiments are taught: one displaying a pre-groove with a triangular transverse section, the other displaying a rectangular transverse section. Both embodiments of the pre-groove are shallower than the ROM data pits, because the photoresist is not fully exposed while the pre-grooves are created.
As discussed above, this partial exposure of the photoresist would generate the triangular-section pre-groove of the first Nakagawa embodiment. However, it is not seen how Nakagawa proposes to generate the rectangular section pre-grooves. In fact, this would be very difficult, if not literally impossible, to accomplish by the means taught. While some light scattering may occur within the photoresist, the exposure cross-section would tend to narrow from the disc surface downward, most likely yielding the typically observed trapezoidal profile.
Possibly recognizing at least the difficulty of creating the second-embodiment shallow, rectangular-section pre-grooves, Nakagawa offers a third embodiment (Column 11, line 66 to column 12, line 36), in which a first beam exposes the photoresist to (hopefully) create the rectangular-section pre-groove, and a second beam then partially exposes the entire R band with the hope of reducing the effective depth of these pre-grooves. It will perhaps be appreciated that this double exposure of the photoresist would, at best, be a very difficult process to control. As with the second embodiment, no teaching is given as to how this might be accomplished.
Furthermore, even if Nakagawa, or one of ordinary skill practicing his teachings, could somehow create disc masters with ROM pits and R band pre-grooves of rectangular cross-section—which is highly unlikely—it would be nearly impossible to replicate such masters, simply because the molded clear replica hybrid discs would adhere strongly to the stampers, which would display millions of features having vertical sides, rather than the slanted sides of the trapezoidal cross-section features normally seen in PR-generated masters.
The depth of the thermally-active recording layer applied over the R band pre-grooves, in Nakagawa's clear replica hybrid discs, must be greater than over the pits already recorded in the ROM band(s). This is to ensure that the resulting pits selectively recorded in the R band(s) will have the same effective optical depth as those already existing in the ROM section, after the spin-coated recording layer that spreads across the entire disc has covered them. Accordingly, Nakagawa suggests that the cross-sectional shape of the respective features will ensure that when the thermally-active recording layer is spin coated over the clear replica hybrid disc, the proper respective depths will be achieved. This would be very unlikely to occur if the pre-grooves have a triangular cross-section. As mentioned above, the manner in which Nakagawa creates the pre-grooves (reduced exposure of the photoresist in those areas) would make it highly unlikely that this method could achieve the desired rectangular cross-section pre-grooves, or that such masters, even if generated, could produce replicas meeting Orange Book specifications. In other words, Nakagawa's simple expedient of allowing the cross-sectional shape of the respective features to cause the desired difference in thermally-active recording layer thickness would probably not succeed.
Ultimately, if Nakagawa's teachings were followed, using modem high speed dyes, not only would tracking be severely compromised, particularly in the hybrid disc R band(s), but also HF detection of the R band pits would likewise be more difficult, as these pits would essentially be squeezed into the narrow pre-grooves and “bulge” radially outwardly from them. The most probable result of following the Nakagawa teachings, therefore, would be hybrid discs that are simply incapable of meeting either Orange or Red Book specifications. Hybrid discs that fail to meet the specs are useless.
Yanagimachi (U.S. Pat. No. 5,696,758), another photoresist mastering method, essentially attempts to follow the Nakagawa teachings. By exposing the photoresist less in the R bands than in the ROM bands, as Nakagawa teaches, and further by employing an exposure level in the ROM lands that is less than the exposure level of the ROM pits, Yanagimachi creates grooves in the ROM area of the clear replica hybrid disc that are narrower and shallower than the pits which they “connect.” When the thermally-active recording layer is then spin coated over the clear replica hybrid disc to create the final hybrid disc, there will be even less of the ROM groove remaining. This is in addition to the problem, as in Nakagawa, that the R band pits and grooves will not readily lend themselves to HF or PP detection. Furthermore, Yanagimachi does not teach how to independently control the width and depth of the ROM grooves, the ROM pits and the R band grooves. One of ordinary skill would know that if a PR generated feature is narrowed by reducing the laser power of a single beam, its depth would be correspondingly reduced. Yet Yanagimachi, which definitely teaches the use of a single beam (Column 6, lines 23–32), offers no assistance in this regard. Thus, the Yanagimachi teachings are, if anything, even less instructive than those of Nakagawa in enabling those of ordinary skill to manufacture hybrid discs that might meet Orange and Red Book specifications.
Ha (U.S. Pat. No. 6,212,158) differs from Yanagimachi primarily in certain parametric values. Indeed, Ha refers to Yanagimachi, pointing out (Column 1: lines 36–43) the difficulty in creating acceptable hybrid discs incorporating the latter's wobbled, depth modulated (ROM area) groove. Accordingly, Ha modifies Yanagimachi with different parametric values, based on a very similar concept, and Ha's claims are essentially “written around” Yanagimachi. But Ha adds nothing material to the foregoing two teachings to facilitate hybrid disc production.
In summary of the prior art known to the applicants, there is no published teaching, either alone or in any reasonable combination, which will enable a practitioner of ordinary skill to reliably, repeatedly and efficiently manufacture hybrid discs meeting Orange Book and Red Book specifications. This may, or may not, be because all publications known to the applicants rely on the photoresist method of mastering the hybrid discs, which, as has been shown above, is extremely difficult, if not impossible, to utilize in producing masters that can be replicated to manufacture hybrid discs meeting the required specifications. Whatever the reasons, it appears that in-spec hybrid discs cannot be commercially manufactured in accordance with the prior art.
Therefore there is a need for a method, an apparatus and resulting pit, land and pre-groove geometry in hybrid disc masters, by which replicated hybrid discs, meeting all applicable manufacturing specifications, may efficiently, rapidly and reliably be manufactured.